As for an electronic medium replacing paper, developments of electronic paper have been recently actively carried out.
The electronic paper has characteristics that the display device thereof is used like paper, and therefore requires properties different from conventional display devices, such as CRT and LCD. For example, required properties thereof are being a reflective a display device as well as having high white reflectance and high contrast ratio, being able to display with high definition, giving the display a memory function, being driven at low voltage, being thin and light, and being inexpensive. Among them, as properties associated with a quality of a display, particularly white reflectivity and contrast ratio close to that of paper, and color display are highly demanded.
Previously, as for a display device for use as electronic paper, for example, proposed are a system using reflecting liquid crystals, a system using electrophoresis, a system using toner migration, and the like. In any of these systems, however, it is very difficult to perform multicolor display with maintaining white reflectivity and contrast ratio. In order to perform multicolor display, color filters are typically provided. When color filters are provided, the color filters themselves absorb light to thereby reduce reflectance. Moreover, use of the color filters requires to divide one pixel into three, red (R), green (G), and blue (B), reflectance of a display device reduces. Along the reduction in the reflectance of the display device, the contrast ratio thereof also reduces. When the white reflectivity and contrast ratio are significantly reduced, the visibility becomes very poor, and therefore it is difficult to use such device as electronic paper.
PTL 1 and PTL 2 each disclose a reflecting color display medium, in an electrophoresis element of which color filters are formed, but it is clear that such display medium cannot provide excellent image quality even when color filters are formed in the display medium of low white reflectivity and a low contrast ratio. Moreover, PTL 3 and PTL 4 each disclose an electrophoresis, with which color display is realized by moving particles, which are tinted in a plurality of colors. Theoretically, use of this method does not lead to a solution for the aforementioned problems, and cannot realize both high white reflectivity and a high contrast ratio.
Meanwhile, as a promising technology for realizing a reflecting display device without providing color filters as described above, there is a system using electrochromic phenomenon. The phenomenon where, as voltage is applied, an oxidation-reduction reaction is reversibly caused depending on the polarity to thereby reversibly change color is called electrochromism. A display device utilizing coloring/bleaching (may also stated as coloring and bleaching hereinafter) of an electrochromic compound which cases this phenomenon is an electrochromic display device. Since this electrochromic display device is a reflecting display device, has a memory effect, and can be driven at low voltage, researches and developments of electrochromic devices have been widely conducted from a development of materials and designing of devices, as a promising option for a display device technology for electronic paper.
However, as the electrochromic display device performs coloring and bleaching using an oxidation-reduction reaction, the electrochromic display device has a disadvantage that a coloring-bleaching response speed is slow. PTL 5 discloses an example, in which an improvement of the coloring-bleaching response speed is attempted by fixing an electrochromic compound adjacent to an electrode. According to the descriptions in PTL 5, although the time required for coloring and bleaching is conventionally about 10 seconds, the time required from colorless to color in blue and the time required from blue to colorless are both improved to about 1 second. However, this is not necessarily sufficient. As for the researches and developments of electrochromic display devices, further improvements of the coloring-bleaching response speed are necessary.
An electrochromic display device can display various colors depending on a structure of an electrochromic compound used as a display material, and therefore it is hoped to be used as a multicolor display device. Especially as the electrochromic display device can reversibly change its colorless state to the color state, a laminate multicolor structure can be realized. In the color display of the laminate structure, it is not necessary to divide one pixel into three, red (R), green (G), and blue (B), as in the conventional technology. Therefore, there are advantages that the reflectance and contrast ratio of the display device are not reduced.
There are several conventional examples of a multicolor display device utilizing such electrochromic display device. For example, PTL 6 discloses a multicolor display device, in which particles of a plurality of electrochromic compounds are laminated. In PTL 6, disclosed is an example of a multicolor display device, in which a plurality of electrochromic compounds, which are high molecular compounds having functional groups, each of which requires different voltage to color, are laminated to form a multicolor display electrochromic compound.
Moreover, PTL 7 discloses a display device, in which multiple electrochromic layers are formed on an electrode to display multiple colors utilizing a difference in voltage or current required for color. In PTL 7, disclosed is an example of a multicolor display device, which colors different colors, and has a display layer formed by laminating or mixing a plurality of electrochromic compounds each require different threshold voltage and electric charge to color.
PTL 8 discloses an electrochromic device in which a full-color display is realized by laminating a plurality of structural units each formed by sandwiching an electrolyte layer containing an electrochromic compound. Moreover, PTL 9 discloses an example of a multicolor display device corresponding to RGB 3 colors, in which a passive matrix panel and active matrix panel are composed of electrochromic elements, in each of which an electrolyte layer containing at least one electrochromic compound is present. Furthermore, PTL 10 discloses that coloring properties and durability are improved by a reversible recording material, in which one, or two or more compounds having the specific structure are contained on surfaces of metal oxide particles. Moreover, PTL 11 discloses an electrochromic compound having a pyridine ring, and represented by the certain structural formula, which can color in yellow and can be bleached. Furthermore, PTL 12 disclosed phthalic acid-based compounds which colors in yellow, magenta, and cyan.
The viologen-based organic electrochromic compounds disclosed in PTL 5, PTL 6, and PTL 7 are compounds having high stability and high durability for repetitive use, but they display colors, such as blue, and green, not 3 primary colors, yellow, magenta, and cyan, required for formation of full color. Moreover, the styryl-based dyes listed in PTL 8, PTL 9, and PTL 10 display excellent yellow, magenta, and cyan, but these dyes have problems in stability of coloring and bleaching, or durability for repetitive use.
Moreover, PTL 11 is related to the compound, which colors in yellow and is bleached, not cyan. The phthalic acid-based compound disclosed in PTL 12 has a problem that such compound has a poor memory function.
As mentioned above, an ideal electrochromic compound for realizing full-color electronic paper has not been provided, and it is desired to provide the material having excellent color tone, durability, and stability.